Lightguides are typically used for guiding light from light sources in various lighting solutions. The lightguides can be used for display lighting (e.g. backlighting, frontlighting), keypad, keyboard and/or button lighting, interior lighting, and exterior lighting, among other applications. Conventional lightguides that are considered as thin may have a thickness of about 0.8 to about 1.0 mm and include micro-optical structures. Such microstructures are often on the order of about 15 microns in height or more, and on the order of about 50 microns or more in one lateral dimension.
FIG. 1 illustrates a lightguide 102 including a plurality of micro-optical structures 106 implemented as surface relief forms on one side of the lightguide 102 for outcoupling 108 light emitted by a light source 104 and transported in the lightguide 102 by total reflection. In the example of FIG. 1, light is outcoupled from the lightguide 102 via a top surface opposite to the bottom surface including the illustrated micro-optical structures 106. The lightguides are often application-specific such that a desired illumination pattern formed by outcoupled light can be precisely obtained.
Now referring to FIG. 2, conventional keypad, keyboard and console lighting typically includes 3-8 LEDs (light-emitting diode) as light sources and a relatively thick lightguide 206 having apertures for each key 202 to enable establishing the associated electrical contact. These designs may include electric circuitry in connection with a dome sheet 208 that includes a plurality of thin metal domes, one per each key or button 202. When pushing the hard/rigid top of the key 202 of a key mat, which makes the flexible and soft, e.g. silicone, plunger 210 of the underlying soft silicone layer 204 to protrude towards the dome sheet 208, the thin metal dome on the sheet 208 is bent and flattened making the intended electrical contact on the surface of the electric circuitry, for example. The domes also provide tactile feeling with a “click” effect and the apertures are needed to provide such function. However, these apertures make it difficult to manage light in order to achieve e.g. uniform keypad lighting. The resulting keypad arrangement may also be impractically thick relative to the initial design goal as the soft material plungers 210 have to be rather thick so as to reach through the thick lightguide 206 and actuate the dome. Additionally rigid plungers cause defects and damages for the dome sheet layers, thus soft silicone plungers are used.
Lightguides may be manufactured according to a number of different processes. Lightguide layer production can be completed by means of continuous roll replication, or “roll-to-roll” replication, for example. Using this manufacturing method bulk material, such as optically transparent plastic film, may be utilized in surface relief replication. Different roll-to-roll methods are prior known and feasible for manufacturing micro-optic surface relief forms (e.g. structures), either refractive or diffractive, for many different applications. Several roll-to-roll methods are available, such as the ones by Reflexite, Avery Dennison, 3M, Epigem, Rolltronics, Polymicro, Printo project, among others. Other suitable production methods may include continuous or discrete casting methods (UV or thermal curing), compression molding, continuous or discrete embossing, such as hard embossing, soft embossing and UV embossing, among others. Melt film can also be used.
One drawback of many roll-to-roll embossing methods is illustrated in FIG. 3. Namely, as two rollers 310, 312, in this example the former of which having a circular cross-section and the latter a circular cross-section excluding two opposite segments, introduce pressure to a flexible lightguide bulk material 314 advanced between them, the bulk material 314 is replicated with an optical surface relief structure of the roller 312, for example. However, it can readily be seen from the visualization of phase 302 how the soft bulk material 314 starts to pile up during the nip pressure (arrows marked with p) and the input speed v1 begins to decrease compared to output speed v0 as the material 314 deforms. The same situation continues in phases 304, 306 until in phase 308 the input speed v1 temporarily again increases due to end of nip pressure segment of the roller 312 and matches speed v0, after which the same cycle of aforesaid phases is repeated. An elongated lightguide may be produced as a result, for example. Such phenomenon creates artifacts in the replicated optical micro-structures as material 314 deformation cannot be properly taken into account in the replication process.